The Mini’s idle has always been a bit rough (lightweight flywheel, uprated cam, solid mount subframe) but lately it’s been worse than usual. Time to figure it out.
Long (looooong) story short, it was a combination of things:
-The new 123ignition electronic distributor does *not* like copper core ignition leads, even with suppressor spark plug caps. The interference causes about 10 degrees of spark scatter at idle. I’ve gone back to the old mechanical distributor with points until I get around to making some carbon-core suppressed leads.
-Rocker clearances. This one’s entirely my fault. I checked them as part of the 10,000km service and set them to the standard 12 thou, completely forgetting that Kent Cams recommends 17 thou for the MD266. 12 thou is too much overlap for a reasonable idle. Set them to 17 thou and magic happens! It’ll idle at 800 rpm!
-Carburettor balance: one of the carburettor’s idle screws was very loose and it would never settle into one place, meaning the idle balance was always a bit off. I tapped the idle screw holes larger and fitted bigger hex socket screws (for easier adjustment) and now the carbs stay in balance.
Things it was not:
-Mixture. I was seeing some wild numbers when the rocker clearances were too tight, but once I’d corrected them it idles at a super stable 14.5:1.
-Lightweight flywheel. I fitted a standard Morris 1300 flywheel, a 22kg hunk of pig iron with a ring gear on the outside. Made no difference to the smoothness, just slowed the engine down. I’d forgotten how difficult it is to swap a flywheel with the engine in place, but luckily I found my old bent 1/2″ ring spanner, the one I bent back in 1998 when working on the Mini van in the carpark.
With that sorted, I now return to adjusting the front alignment (over and over again) to dial out the weird twitchiness that started after I installed the Quaife ATB. The good folks over at TheMiniForum.co.uk mostly confirm what Quaife say: that during normal driving you shouldn’t know it’s there, apart from heavier steering.
We’ve confirmed that there *is* a break in period (despite what Quaife say) where the fresh new diff loosens up, and it is indeed behaving a little better, though there’s still a disturbing weave to one side or the other when balancing the accelerator between drive and coast. More fettling needed.